The concept of a topocatheter extension added to a standard angiographic catheter involves the production of a high strength thin walled tapered tube attached to the end of a standard catheter. Suction on the catheter causes the thin wall portion to collapse and invert itself until it lines the distal end of the catheter. Flow out of the catheter causes the thin wall to valve shut and evert again. When the thin wall everts it rolls onto the inside of the vessel wall with no relative wall motion hence no friction. The rolling wall is advanced by the pressure of the fluid inside so that the extension enters and follows small or tortuous vessels with ease. Catheter extensions have now been made of reconstituted collagen sealed with a polyurethane dip coat to improve the strength and compliance to inversion and eversion while reducing the possibility of blocking blood flow by balooning, extensions up to 15 cm long on number 5F have successfully entered cerebral vasculature in the monkey. Testing of these catheters have shown them to require 900 mm of Hg rupture them but eversion is accomplished below 200 mm of Hg. Tests of other collagens have indicated that our present methods are adequate and EM studies confirm a multilayer interwoven fibrous structure and good adherence to the polyurethane. Application to the therapy of AV malformations in the Radiology Department is anticipated.